srcset-w-reselect-on-resize.html

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This WPT test may be referenced by the following Test IDs:
- /html/semantics/embedded-content/the-img-element/srcset/srcset-w-reselect-on-resize.html - WPT Dashboard Interop Dashboard

<!doctype html>

<title>srcset w-descriptor reselects on viewport resize</title>

<link rel="help" href="https://html.spec.whatwg.org/multipage/images.html#reacting-to-environment-changes">

<link rel="help" href="https://crbug.com/41385706">

<script src="/resources/testharness.js"></script>

<script src="/resources/testharnessreport.js"></script>

<script src="/common/utils.js"></script>

<div id=log></div>

<script>

// Tests that an <img> with srcset w-descriptors correctly re-selects the

// image source when the viewport (iframe) is resized.

function resolveUrl(url) {

  const a = document.createElement('a');

  a.href = url;

  return a.href;

function createIframe(width) {

  const id = token();

  return new Promise(resolvePromise => {

    const iframe = document.createElement('iframe');

    iframe.style.width = width + 'px';

    iframe.srcdoc = `<!doctype html>

                     <img srcset="/images/green-1x1.png?200w-${id} 200w,

                                  /images/green-2x2.png?800w-${id} 800w"

                          sizes="100vw">`;

    iframe.onload = () => resolvePromise({iframe, id});

    document.body.appendChild(iframe);

});

promise_test(async t => {

  const {iframe, id} = await createIframe(100);

  const img = iframe.contentDocument.querySelector('img');

  const narrow = resolveUrl(`/images/green-1x1.png?200w-${id}`);

  const wide = resolveUrl(`/images/green-2x2.png?800w-${id}`);

  assert_equals(img.currentSrc, narrow, 'narrow viewport should select 200w');

  // Set up load listener before resizing so we catch the re-selection.

  const loaded = new Promise(resolve =>

      img.addEventListener('load', resolve, {once: true}));

  iframe.style.width = '1000px';

  await loaded;

  assert_equals(img.currentSrc, wide, 'after widening should select 800w');

}, 'srcset w-descriptor: narrow to wide selects higher-density candidate');

</script>